Method for updating a network allocation vector (NAV) in wireless LAN system and apparatus therefor

ABSTRACT

A method for updating a network allocation vector (NAV) by a station (STA) in a wireless LAN system according to an embodiment of the present invention comprises the steps of: receiving a frame; and when the duration of the frame is greater than the value of an NAV of the STA, updating the NAV of the STA, wherein the duration is indicated by each of a signal (SIG) field of a physical layer and a header of an MAC layer in the frame, the duration indicated by the physical layer has a larger granularity of a time unit than that of the duration indicated by the MAC layer, and the STA updates the NAV through the duration having the larger granularity, indicated by the physical layer, when the duration indicated by the MAC layer cannot be obtained.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/766,773, filed on Apr. 6, 2018, now U.S. Pat. No. 10,492,224, whichis the National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2016/012089, filed on Oct. 26, 2016, which claimsthe benefit of U.S. Provisional Application No. 62/246,153, filed onOct. 26, 2015, and 62/299,020, filed on Feb. 24, 2016, the contents ofwhich are all hereby incorporated by reference herein in theirentireties.

TECHNICAL FIELD

The present invention relates to a method and apparatus for updating anetwork allocation vector (NAV) in a wireless LAN system, and moreparticularly, to a method of updating NAV based on a received frame andapparatus therefor.

BACKGROUND ART

Standards for Wireless Local Area Network (WLAN) technology have beendeveloped as Institute of Electrical and Electronics Engineers (IEEE)802.11 standards. IEEE 802.11a and b use an unlicensed band at 2.4 GHzor 5 GHz. IEEE 802.11b provides a transmission rate of 11 Mbps and IEEE802.11a provides a transmission rate of 54 Mbps. IEEE 802.11g provides atransmission rate of 54 Mbps by applying Orthogonal Frequency DivisionMultiplexing (OFDM) at 2.4 GHz. IEEE 802.11n provides a transmissionrate of 300 Mbps for four spatial streams by applying Multiple InputMultiple Output (MIMO)-OFDM. IEEE 802.11n supports a channel bandwidthof up to 40 MHz and, in this case, provides a transmission rate of 600Mbps.

The above-described WLAN standards have evolved into IEEE 802.11ac thatuses a bandwidth of up to 160 MHz and supports a transmission rate of upto 1 Gbits/s for 8 spatial streams and IEEE 802.11ax standards are underdiscussion.

DISCLOSURE OF THE INVENTION Technical Task

The technical task of the present invention is to provide a method andapparatus for an STA, which has received a frame indicating a durationon each of a physical layer and a MAC layer, to update NAV in a wirelessLAN system.

The present invention is not limited to the above technical problems andother technical objects may be inferred from embodiments of the presentinvention.

Technical Solutions

In one technical aspect of the present invention, provided herein is amethod of updating a network allocation vector (NAV) by a station (STA)in a wireless LAN system, including receiving a frame and updating theNAV of the STA if a duration of the frame is greater than a NAV value ofthe STA, wherein the duration is indicated by each of a signal (SIG)field of a physical layer of the frame and a header of a MAC layer,wherein the duration indicated on the physical layer has a greatergranularity of a time unit than that of the duration indicated on theMAC layer, and wherein the STA performs the NAV update through aduration of the greater granularity indicated on the physical layer incase where the STA does not obtain the duration indicated on the MAClayer.

In another technical aspect of the present invention, provided herein isa station (STA), including a receiver receiving a frame and a processorupdating the NAV of the STA if a duration of the frame is greater thanan NAV value of the STA, wherein the duration is indicated by each of asignal (SIG) field of a physical layer of the frame and a header of aMAC layer, wherein the duration indicated on the physical layer has agreater granularity of a time unit than that of the duration indicatedon the MAC layer, and wherein the processor performs the NAV updatethrough a duration of the greater granularity indicated on the physicallayer in case where the processor does not obtain the duration indicatedon the MAC layer.

If the duration indicated on the MAC layer is obtainable, the STA mayignore the duration indicated on the physical layer and perform the NAVupdate through the duration indicated on the MAC layer.

The case where the STA does not obtain the duration indicated on the MAClayer may include one of a case that the STA fails to decode all MPDUs(MAC protocol data units) included in the frame, a case that thereceived frame is an uplink frame, a case that a header of the MAC layeris a short MAC header type, and a case that the STA makes transition toa doze mode for power saving after decoding the signal field.

In updating the NAV, the STA may select and update one of a plurality ofNAVs maintained by the STA depending on whether the frame is receivedfrom a BSS (basic service set) where the STA belongs and the pluralityof NAVs may include an intra-BSS NAV and an inter-BSS NAV.

If the frame is an OBSS (overlapping BSS) frame and a power of the frameis smaller than an OBSS PD (packet detection) level, an update of theinter-BSS NAV may not be performed even if the duration of the frameexceeds a value of the inter-BSS NAV.

The OBSS PD level may be provided for a spatial reuse and have a valuegreater than a CCA (clear channel assessment) threshold applied to anintra-BSS frame.

The NAV update is performed if the frame is not intended to the STA. TheSTA determines that the frame is not intended to the STA if an RA(recipient address) of the frame is not identical to a MAC address ofthe STA, if an AID (association identifier) of the STA is not includedin the signal field, or if a BSS (basic service set) color included inthe signal field is not identical to a color of a BSS where the STAbelongs.

Advantageous Effects

According to one embodiment of the present invention, an STA havingreceived a frame indicating a duration on each of a physical layer and aMAC layer updates NAV by preferentially considering duration configuredwith relatively fine granularity, whereby the NAV can be managed moreaccurately and efficiently.

Other technical effects in addition to the above-described effects maybe inferred from embodiments of the present invention.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of a configuration of a wireless LANsystem.

FIG. 2 illustrates another example of a configuration of a wireless LANsystem.

FIG. 3 illustrates a general link setup procedure.

FIG. 4 illustrates a backoff procedure.

FIG. 5 is an explanatory diagram of a hidden node and an exposed node.

FIG. 6 is an explanatory diagram of RTS and CTS.

FIGS. 7 to 9 are explanatory diagrams of operation of an STA that hasreceived TIM.

FIG. 10 is an explanatory diagram of an exemplary frame structure usedin an IEEE 802.11 system.

FIG. 11 illustrates a contention free (CF)-END frame.

FIG. 12 illustrates an example of an HE PPDU.

FIG. 13 illustrates another example of the HE PPDU.

FIG. 14 illustrates another example of the HE PPDU.

FIG. 15 illustrates another example of the HE PPDU.

FIG. 16 illustrates another example of the HE PPDU.

FIGS. 17 and 18 illustrating an HE-SIG B padding method.

FIG. 19 is an explanatory diagram of uplink multi-user transmissionaccording to an embodiment of the present invention.

FIG. 20 illustrates a trigger frame format according to an embodiment ofthe present invention.

FIG. 21 illustrates an example of NAV setting.

FIG. 22 and FIG. 23 are flowcharts of an NAV updating method accordingto one embodiment of the present invention.

FIG. 24 is a diagram to describe an apparatus according to oneembodiment of the present invention.

BEST MODE FOR INVENTION

Reference will now be made in detail to the exemplary embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the present invention.

The following detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details. In some instances, knownstructures and devices are omitted or are shown in block diagram form,focusing on important features of the structures and devices, so as notto obscure the concept of the present invention.

As described before, the following description is given of a method andapparatus for increasing a spatial reuse rate in a Wireless Local AreaNetwork (WLAN) system. To do so, a WLAN system to which the presentinvention is applied will first be described in detail.

FIG. 1 is a diagram illustrating an exemplary configuration of a WLANsystem.

As illustrated in FIG. 1, the WLAN system includes at least one BasicService Set (BSS). The BSS is a set of STAs that are able to communicatewith each other by successfully performing synchronization.

An STA is a logical entity including a physical layer interface betweena Media Access Control (MAC) layer and a wireless medium. The STA mayinclude an AP and a non-AP STA. Among STAs, a portable terminalmanipulated by a user is the non-AP STA. If a terminal is simply calledan STA, the STA refers to the non-AP STA. The non-AP STA may also bereferred to as a terminal, a Wireless Transmit/Receive Unit (WTRU), aUser Equipment (UE), a Mobile Station (MS), a mobile terminal, or amobile subscriber unit.

The AP is an entity that provides access to a Distribution System (DS)to an associated STA through a wireless medium. The AP may also bereferred to as a centralized controller, a Base Station (BS), a Node-B,a Base Transceiver System (BTS), or a site controller.

The BSS may be divided into an infrastructure BSS and an Independent BSS(IBSS).

The BSS illustrated in FIG. 1 is the IBSS. The IBSS refers to a BSS thatdoes not include an AP. Since the IBSS does not include the AP, the IBSSis not allowed to access to the DS and thus forms a self-containednetwork.

FIG. 2 is a diagram illustrating another exemplary configuration of aWLAN system.

BSSs illustrated in FIG. 2 are infrastructure BSSs. Each infrastructureBSS includes one or more STAs and one or more APs. In the infrastructureBSS, communication between non-AP STAs is basically conducted via an AP.However, if a direct link is established between the non-AP STAs, directcommunication between the non-AP STAs may be performed.

As illustrated in FIG. 2, the multiple infrastructure BSSs may beinterconnected via a DS. The BSSs interconnected via the DS are calledan Extended Service Set (ESS). STAs included in the ESS may communicatewith each other and a non-AP STA within the same ESS may move from oneBSS to another BSS while seamlessly performing communication.

The DS is a mechanism that connects a plurality of APs to one another.The DS is not necessarily a network. As long as it provides adistribution service, the DS is not limited to any specific form. Forexample, the DS may be a wireless network such as a mesh network or maybe a physical structure that connects APs to one another.

Layer Architecture

An operation of a STA in a WLAN system may be described from theperspective of a layer architecture. A processor may implement the layerarchitecture in terms of device configuration. The STA may have aplurality of layers. For example, the 802.11 standards mainly deal witha MAC sublayer and a PHY layer on a Data Link Layer (DLL). The PHY layermay include a Physical Layer Convergence Protocol (PLCP) entity, aPhysical Medium Dependent (PMD) entity, and the like. Each of the MACsublayer and the PHY layer conceptually includes management entitiescalled MAC sublayer Management Entity (MLME) and Physical LayerManagement Entity (PLME). These entities provide layer managementservice interfaces through which a layer management function isexecuted.

To provide a correct MAC operation, a Station Management Entity (SME)resides in each STA. The SME is a layer independent entity which may beperceived as being present in a separate management plane or as beingoff to the side. While specific functions of the SME are not describedin detail herein, the SME may be responsible for collectinglayer-dependent states from various Layer Management Entities (LMEs) andsetting layer-specific parameters to similar values. The SME may executethese functions and implement a standard management protocol on behalfof general system management entities.

The above-described entities interact with one another in variousmanners. For example, the entities may interact with one another byexchanging GET/SET primitives between them. A primitive refers to a setof elements or parameters related to a specific purpose. AnXX-GET.request primitive is used to request a predetermined MIBattribute value (management information-based attribute information). AnXX-GET.confirm primitive is used to return an appropriate MIB attributeinformation value when the Status field indicates “Success” and toreturn an error indication in the Status field when the Status fielddoes not indicate “Success”. An XX-SET.request primitive is used torequest setting of an indicated MIB attribute to a predetermined value.When the MIB attribute indicates a specific operation, the MIB attributerequests the specific operation to be performed. An XX-SET.confirmprimitive is used to confirm that the indicated MIB attribute has beenset to a requested value when the Status field indicates “Success” andto return an error condition in the Status field when the Status fielddoes not indicate “Success”. When the MIB attribute indicates a specificoperation, it confirms that the operation has been performed.

Also, the MLME and the SME may exchange various MLME_GET/SET primitivesthrough an MLME Service Access Point (MLME_SAP). In addition, variousPLME_GET/SET primitives may be exchanged between the PLME and the SMEthrough a PLME_SAP, and exchanged between the MLME and the PLME throughan MLME-PLME_SAP.

Link Setup Process

FIG. 3 is a flowchart explaining a general link setup process accordingto an exemplary embodiment of the present invention.

In order to allow a STA to establish link setup on the network as wellas to transmit/receive data over the network, the STA must perform suchlink setup through processes of network discovery, authentication, andassociation, and must establish association and perform securityauthentication. The link setup process may also be referred to as asession initiation process or a session setup process. In addition, anassociation step is a generic term for discovery, authentication,association, and security setup steps of the link setup process.

Link setup process is described referring to FIG. 3.

In step S510, STA may perform the network discovery action. The networkdiscovery action may include the STA scanning action. That is, STA mustsearch for an available network so as to access the network. The STAmust identify a compatible network before participating in a wirelessnetwork. Here, the process for identifying the network contained in aspecific region is referred to as a scanning process.

The scanning scheme is classified into active scanning and passivescanning.

FIG. 3 is a flowchart illustrating a network discovery action includingan active scanning process. In the case of the active scanning, a STAconfigured to perform scanning transmits a probe request frame and waitsfor a response to the probe request frame, such that the STA can movebetween channels and at the same time can determine which Access Point(AP) is present in a peripheral region. A responder transmits a proberesponse frame, acting as a response to the probe request frame, to theSTA having transmitted the probe request frame. In this case, theresponder may be a STA that has finally transmitted a beacon frame in aBSS of the scanned channel. In BSS, since the AP transmits the beaconframe, the AP operates as a responder. In IBSS, since STAs of the IBSSsequentially transmit the beacon frame, the responder is not constant.For example, the STA, that has transmitted the probe request frame atChannel #1 and has received the probe response frame at Channel #1,stores BSS-associated information contained in the received proberesponse frame, and moves to the next channel (for example, Channel #2),such that the STA may perform scanning using the same method (i.e.,probe request/response transmission/reception at Channel #2).

Although not shown in FIG. 3, the scanning action may also be carriedout using passive scanning A STA configured to perform scanning in thepassive scanning mode waits for a beacon frame while simultaneouslymoving from one channel to another channel. The beacon frame is one ofmanagement frames in IEEE 802.11, indicates the presence of a wirelessnetwork, enables the STA performing scanning to search for the wirelessnetwork, and is periodically transmitted in a manner that the STA canparticipate in the wireless network. In BSS, the AP is configured toperiodically transmit the beacon frame. In IBSS, STAs of the IBSS areconfigured to sequentially transmit the beacon frame. If each STA forscanning receives the beacon frame, the STA stores BSS informationcontained in the beacon frame, and moves to another channel and recordsbeacon frame information at each channel. The STA having received thebeacon frame stores BSS-associated information contained in the receivedbeacon frame, moves to the next channel, and thus performs scanningusing the same method.

In comparison between the active scanning and the passive scanning, theactive scanning is more advantageous than the passive scanning in termsof delay and power consumption.

After the STA discovers the network, the STA may perform theauthentication process in step S520. The authentication process may bereferred to as a first authentication process in such a manner that theauthentication process can be clearly distinguished from the securitysetup process of step S540.

The authentication process may include transmitting an authenticationrequest frame to an AP by the STA, and transmitting an authenticationresponse frame to the STA by the AP in response to the authenticationrequest frame. The authentication frame used for authenticationrequest/response may correspond to a management frame.

The authentication frame may include an authentication algorithm number,an authentication transaction sequence number, a state code, a challengetext, a Robust Security Network (RSN), a Finite Cyclic Group (FCG), etc.The above-mentioned information contained in the authentication framemay correspond to some parts of information capable of being containedin the authentication request/response frame, may be replaced with otherinformation, or may include additional information.

The STA may transmit the authentication request frame to the AP. The APmay decide whether to authenticate the corresponding STA on the basis ofinformation contained in the received authentication request frame. TheAP may provide the authentication result to the STA through theauthentication response frame.

After the STA has been successfully authenticated, the associationprocess may be carried out in step S530. The association process mayinvolve transmitting an association request frame to the AP by the STA,and transmitting an association response frame to the STA by the AP inresponse to the association request frame.

For example, the association request frame may include informationassociated with various capabilities, a beacon listen interval, aService Set Identifier (SSID), supported rates, supported channels, RSN,mobility domain, supported operating classes, a TIM (Traffic IndicationMap) broadcast request, interworking service capability, etc.

For example, the association response frame may include informationassociated with various capabilities, a state code, an Association ID(AID), supported rates, an Enhanced Distributed Channel Access (EDCA)parameter set, a Received Channel Power Indicator (RCPI), a ReceivedSignal to Noise Indicator (RSNI), mobility domain, a timeout interval(association comeback time), an overlapping BSS scan parameter, a TIMbroadcast response, a Quality of Service (QoS) map, etc.

The above-mentioned information may correspond to some parts ofinformation capable of being contained in the associationrequest/response frame, may be replaced with other information, or mayinclude additional information.

After the STA has been successfully associated with the network, asecurity setup process may be carried out in step S540. The securitysetup process of Step S540 may be referred to as an authenticationprocess based on Robust Security Network Association (RSNA)request/response. The authentication process of step S520 may bereferred to as a first authentication process, and the security setupprocess of Step S540 may also be simply referred to as an authenticationprocess.

For example, the security setup process of Step S540 may include aprivate key setup process through 4-way handshaking based on anExtensible Authentication Protocol over LAN (EAPOL) frame. In addition,the security setup process may also be carried out according to othersecurity schemes not defined in IEEE 802.11 standards.

Medium Access Mechanism

In the IEEE 802.11-based WLAN system, a basic access mechanism of MediumAccess Control (MAC) is a Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is referred to as aDistributed Coordination Function (DCF) of IEEE 802.11 MAC, andbasically includes a “Listen Before Talk” access mechanism. Inaccordance with the above-mentioned access mechanism, the AP and/or STAmay perform Clear Channel Assessment (CCA) for sensing an RF channel ormedium during a predetermined time interval [for example, DCFInter-Frame Space (DIFS)], prior to data transmission. If it isdetermined that the medium is in the idle state, frame transmissionthrough the corresponding medium begins. On the other hand, if it isdetermined that the medium is in the occupied state, the correspondingAP and/or STA does not start its own transmission, establishes a delaytime (for example, a random backoff period) for medium access, andattempts to start frame transmission after waiting for a predeterminedtime. Through application of a random backoff period, it is expectedthat multiple STAs will attempt to start frame transmission afterwaiting for different times, resulting in minimum collision.

In addition, IEEE 802.11 MAC protocol provides a Hybrid CoordinationFunction (HCF). HCF is based on DCF and Point Coordination Function(PCF). PCF refers to the polling-based synchronous access scheme inwhich periodic polling is executed in a manner that all reception (Rx)APs and/or STAs can receive the data frame. In addition, HCF includesEnhanced Distributed Channel Access (EDCA) and HCF Controlled ChannelAccess (HCCA). EDCA is achieved when the access scheme provided from aprovider to a plurality of users is contention-based. HCCA is achievedby the contention-free-based channel access scheme based on the pollingmechanism. In addition, HCF includes a medium access mechanism forimproving Quality of Service (QoS) of WLAN, and may transmit QoS data inboth a Contention Period (CP) and a Contention Free Period (CFP).

FIG. 4 is a conceptual diagram illustrating a backoff process.

Operations based on a random backoff period will hereinafter bedescribed with reference to FIG. 4. If the occupy- or busy-state mediumis shifted to an idle state, several STAs may attempt to transmit data(or frame). As a method for implementing a minimum number of collisions,each STA selects a random backoff count, waits for a slot timecorresponding to the selected backoff count, and then attempts to startdata transmission. The random backoff count has a value of a PacketNumber (PN), and may be set to one of 0 to CW values. In this case, CWrefers to a Contention Window parameter value. Although an initial valueof the CW parameter is denoted by CW min, the initial value may bedoubled in case of a transmission failure (for example, in the case inwhich ACK of the transmission frame is not received). If the CWparameter value is denoted by CW max, CW max is maintained until datatransmission is successful, and at the same time it is possible toattempt to start data transmission. If data transmission was successful,the CW parameter value is reset to CW min Preferably, CW, CW min, and CWmax are set to 2^(n)−1 (where n=0, 1, 2, . . . ).

If the random backoff process starts operation, the STA continuouslymon^(i)tors the medium while counting down the backoff slot in responseto the decided backoff count value. If the medium is monitored as theoccupied state, the countdown stops and waits for a predetermined time.If the medium is in the idle state, the remaining countdown restarts.

As shown in the example of FIG. 4, if a packet to be transmitted to MACof STA3 arrives at the STA3, the STA3 determines whether the medium isin the idle state during the DIFS, and may directly start frametransmission. In the meantime, the remaining STAs monitor whether themedium is in the busy state, and wait for a predetermined time. Duringthe predetermined time, data to be transmitted may occur in each ofSTA1, STA2, and STA5. If the medium is in the idle state, each STA waitsfor the DIFS time and then performs countdown of the backoff slot inresponse to a random backoff count value selected by each STA. Theexample of FIG. 4 shows that STA2 selects the lowest backoff count valueand STA1 selects the highest backoff count value. That is, after STA2finishes backoff counting, the residual backoff time of STA5 at a frametransmission start time is shorter than the residual backoff time ofSTA1. Each of STA1 and STA5 temporarily stops countdown while STA2occupies the medium, and waits for a predetermined time. If occupying ofthe STA2 is finished and the medium re-enters the idle state, each ofSTA1 and STA5 waits for a predetermined time DIFS, and restarts backoffcounting. That is, after the remaining backoff slot as long as theresidual backoff time is counted down, frame transmission may startoperation. Since the residual backoff time of STA5 is shorter than thatof STA1, STA5 starts frame transmission. Meanwhile, data to betransmitted may occur in STA4 while STA2 occupies the medium. In thiscase, if the medium is in the idle state, STA4 waits for the DIFS time,performs countdown in response to the random backoff count valueselected by the STA4, and then starts frame transmission. FIG. 4exemplarily shows the case in which the residual backoff time of STA5 isidentical to the random backoff count value of STA4 by chance. In thiscase, an unexpected collision may occur between STA4 and STA5. If thecollision occurs between STA4 and STA5, each of STA4 and STA5 does notreceive ACK, resulting in the occurrence of a failure in datatransmission. In this case, each of STA4 and STA5 increases the CW valuetwo times, and STA4 or STA5 may select a random backoff count value andthen perform countdown. Meanwhile, STA1 waits for a predetermined timewhile the medium is in the occupied state due to transmission of STA4and STA5. In this case, if the medium is in the idle state, STA1 waitsfor the DIFS time, and then starts frame transmission after lapse of theresidual backoff time.

STA Sensing Operation

As described above, the CSMA/CA mechanism includes not only a physicalcarrier sensing mechanism in which the AP and/or STA can directly sensethe medium, but also a virtual carrier sensing mechanism. The virtualcarrier sensing mechanism can solve some problems (such as a hidden nodeproblem) encountered in the medium access. For the virtual carriersensing, MAC of the WLAN system can utilize a Network Allocation Vector(NAV). In more detail, by means of the NAV value, the AP and/or STA,each of which currently uses the medium or has authority to use themedium, may inform another AP and/or another STA for the remaining timein which the medium is available. Accordingly, the NAV value maycorrespond to a reserved time in which the medium will be used by the APand/or STA configured to transmit the corresponding frame. A STA havingreceived the NAV value may prohibit medium access (or channel access)during the corresponding reserved time. For example, NAV may be setaccording to the value of a ‘duration’ field of the MAC header of theframe.

The robust collision detect mechanism has been proposed to reduce theprobability of such collision, and as such a detailed descriptionthereof will hereinafter be described with reference to FIGS. 7 and 8.Although an actual carrier sensing range is different from atransmission range, it is assumed that the actual carrier sensing rangeis identical to the transmission range for convenience of descriptionand better understanding of the present invention.

FIG. 5 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 5(a) exemplarily shows the hidden node. In FIG. 5(a), STA Acommunicates with STA B, and STA C has information to be transmitted. InFIG. 5(a), STA C may determine that the medium is in the idle state whenperforming carrier sensing before transmitting data to STA B, under thecondition that STA A transmits information to STA B. Since transmissionof STA A (i.e., occupied medium) may not be detected at the location ofSTA C, it is determined that the medium is in the idle state. In thiscase, STA B simultaneously receives information of STA A and informationof STA C, resulting in the occurrence of collision. Here, STA A may beconsidered as a hidden node of STA C.

FIG. 5(b) exemplarily shows an exposed node. In FIG. 5(b), under thecondition that STA B transmits data to STA A, STA C has information tobe transmitted to STA D. If STA C performs carrier sensing, it isdetermined that the medium is occupied due to transmission of STA B.Therefore, although STA C has information to be transmitted to STA D,the medium-occupied state is sensed, such that the STA C must wait for apredetermined time (i.e., standby mode) until the medium is in the idlestate. However, since STA A is actually located out of the transmissionrange of STA C, transmission from STA C may not collide withtransmission from STA B from the viewpoint of STA A, such that STA Cunnecessarily enters the standby mode until STA B stops transmission.Here, STA C is referred to as an exposed node of STA B.

FIG. 6 is a conceptual diagram illustrating Request To Send (RTS) andClear To Send (CTS).

In order to efficiently utilize the collision avoidance mechanism underthe above-mentioned situation of FIG. 5, it is possible to use a shortsignaling packet such as RTS and CTS. RTS/CTS between two STAs may beoverheard by peripheral STA(s), such that the peripheral STA(s) mayconsider whether information is communicated between the two STAs. Forexample, if STA to be used for data transmission transmits the RTS frameto the STA having received data, the STA having received data transmitsthe CTS frame to peripheral STAs, and may inform the peripheral STAsthat the STA is going to receive data.

FIG. 6(a) exemplarily shows the method for solving problems of thehidden node. In FIG. 6(a), it is assumed that each of STA A and STA C isready to transmit data to STA B. If STA A transmits RTS to STA B, STA Btransmits CTS to each of STA A and STA C located in the vicinity of theSTA B. As a result, STA C must wait for a predetermined time until STA Aand STA B stop data transmission, such that collision is prevented fromoccurring.

FIG. 6(b) exemplarily shows the method for solving problems of theexposed node. STA C performs overhearing of RTS/CTS transmission betweenSTA A and STA B, such that STA C may determine no collision although ittransmits data to another STA (for example, STA D). That is, STA Btransmits an RTS to all peripheral STAs, and only STA A having data tobe actually transmitted can transmit a CTS. STA C receives only the RTSand does not receive the CTS of STA A, such that it can be recognizedthat STA A is located outside of the carrier sensing range of STA C.

Power Management

As described above, the WLAN system has to perform channel sensingbefore STA performs data transmission/reception. The operation of alwayssensing the channel causes persistent power consumption of the STA.There is not much difference in power consumption between the Reception(Rx) state and the Transmission (Tx) state. Continuous maintenance ofthe Rx state may cause large load to a power-limited STA (i.e., STAoperated by a battery). Therefore, if STA maintains the Rx standby modeso as to persistently sense the channel, power is inefficiently consumedwithout special advantages in terms of WLAN throughput. In order tosolve the above-mentioned problem, the WLAN system supports a PowerManagement (PM) mode of the STA.

The PM mode of the STA is classified into an active mode and a PowerSave (PS) mode. The STA is basically operated in the active mode. TheSTA operating in the active mode maintains an awake state. If the STA isin the awake state, the STA may normally operate such that it canperform frame transmission/reception, channel scanning, or the like. Onthe other hand, STA operating in the PS mode is configured to switchfrom the doze state to the awake state or vice versa. STA operating inthe sleep state is operated with minimum power, and the STA does notperform frame transmission/reception and channel scanning.

The amount of power consumption is reduced in proportion to a specifictime in which the STA stays in the sleep state, such that the STAoperation time is increased in response to the reduced powerconsumption. However, it is impossible to transmit or receive the framein the sleep state, such that the STA cannot mandatorily operate for along period of time. If there is a frame to be transmitted to the AP,the STA operating in the sleep state is switched to the awake state,such that it can transmit/receive the frame in the awake state. On theother hand, if the AP has a frame to be transmitted to the STA, thesleep-state STA is unable to receive the frame and cannot recognize thepresence of a frame to be received. Accordingly, STA may need to switchto the awake state according to a specific period in order to recognizethe presence or absence of a frame to be transmitted to the STA (or inorder to receive a signal indicating the presence of the frame on theassumption that the presence of the frame to be transmitted to the STAis decided).

The AP may transmit a beacon frame to STAs in a BSS at predeterminedintervals. The beacon frame may include a traffic indication map (TIM)information element. The TIM information element may include informationindicating that the AP has buffered traffic for STAs associatedtherewith and will transmit frames. TIM elements include a TIM used toindicate a unicast frame and a delivery traffic indication map (DTIM)used to indicate a multicast or broadcast frame.

FIGS. 7 to 9 are conceptual diagrams illustrating detailed operations ofthe STA having received a Traffic Indication Map (TIM).

Referring to FIG. 7, STA is switched from the sleep state to the awakestate so as to receive the beacon frame including a TIM from the AP. STAinterprets the received TIM element such that it can recognize thepresence or absence of buffered traffic to be transmitted to the STA.After STA contends with other STAs to access the medium for PS-Pollframe transmission, the STA may transmit the PS-Poll frame forrequesting data frame transmission to the AP. The AP having received thePS-Poll frame transmitted by the STA may transmit the frame to the STA.STA may receive a data frame and then transmit an ACK frame to the AP inresponse to the received data frame. Thereafter, the STA may re-enterthe sleep state.

As can be seen from FIG. 7, the AP may operate according to theimmediate response scheme, such that the AP receives the PS-Poll framefrom the STA and transmits the data frame after lapse of a predeterminedtime [for example, Short Inter-Frame Space (SIFS)]. In contrast, the APhaving received the PS-Poll frame does not prepare a data frame to betransmitted to the STA during the SIFS time, such that the AP mayoperate according to the deferred response scheme, and as such adetailed description thereof will hereinafter be described withreference to FIG. 8.

The STA operations of FIG. 8 in which the STA is switched from the sleepstate to the awake state, receives a TIM from the AP, and transmits thePS-Poll frame to the AP through contention are identical to those ofFIG. 7. If the AP having received the PS-Poll frame does not prepare adata frame during the SIFS time, the AP may transmit the ACK frame tothe STA instead of transmitting the data frame. If the data frame isprepared after transmission of the ACK frame, the AP may transmit thedata frame to the STA after completion of such contending. STA maytransmit the ACK frame indicating successful reception of a data frameto the AP, and may be shifted to the sleep state.

FIG. 9 shows the exemplary case in which AP transmits DTIM. STAs may beswitched from the sleep state to the awake state so as to receive thebeacon frame including a DTIM element from the AP. STAs may recognizethat multicast/broadcast frame(s) will be transmitted through thereceived DTIM. After transmission of the beacon frame including theDTIM, AP may directly transmit data (i.e., multicast/broadcast frame)without transmitting/receiving the PS-Poll frame. While STAscontinuously maintains the awake state after reception of the beaconframe including the DTIM, the STAs may receive data, and then switch tothe sleep state after completion of data reception.

Frame Structure

FIG. 10 is an explanatory diagram of an exemplary frame structure usedin an IEEE 802.11 system.

A PPDU (Physical Layer Protocol Data Unit) frame format may include anSTF (Short Training Field), an LTF (Long Training Field), a SIG (SIGNAL)field and a data field. The most basic (e.g., non-HT (High Throughput))PPDU frame format may include only an L-STF (Legacy-STF), an L-LTF(Legacy-LTF), a SIG field and a data field.

The STF is a signal for signal detection, AGC (Automatic Gain Control),diversity selection, accurate time synchronization, etc., and the LTF isa signal for channel estimation, frequency error estimation, etc. TheSTF and LTF may be collectively called a PLCP preamble. The PLCPpreamble may be regarded as a signal for OFDM physical layersynchronization and channel estimation.

The SIG field may include a RATE field and a LENGTH field. The RATEfield may include information about modulation and coding rates of data.The LENGTH field may include information about the length of data. Inaddition, the SIG field may include a parity bit, a SIG TAIL bit, etc.

The data field may include a SERVICE field, a PSDU (Physical layerService Data Unit) and a PPDU TAIL bit. The data field may also includepadding bits as necessary. Some bits of the SERVICE field may be usedfor synchronization of a descrambler at a receiving end. The PSDUcorresponds to an MPDU (MAC Protocol Data Unit) defined in the MAC layerand may include data generated/used in a higher layer. The PPDU TAIL bitmay be used to return an encoder to state 0. The padding bits may beused to adjust the length of the data field to a predetermined unit.

The MPDU is defined depending on various MAC frame formats, and a basicMAC frame includes a MAC header, a frame body and an FCS (Frame CheckSequence). The MAC frame may be composed of the MPDU andtransmitted/received through PSDU of a data part of the PPDU frameformat.

The MAC header includes a frame control field, a duration/ID field, anaddress field, etc. The frame control field may include controlinformation necessary for frame transmission/reception. The duration/IDfield may be set to a time to transmit a relevant a relevant frame.

The duration/ID field included in the MAC header may be set to a 16-bitlength (e.g., B0 to B15). Content included in the duration/ID field maydepend on frame type and sub-type, whether transmission is performed fora CFP (contention free period), QoS capability of a transmission STA andthe like. (i) In a control frame corresponding to a sub-type of PS-Poll,the duration/ID field may include the AID of the transmission STA (e.g.,through 14 LSBs) and 2 MSBs may be set to 1. (ii) In frames transmittedby a PC (point coordinator) or a non-QoS STA for a CFP, the duration/IDfield may be set to a fixed value (e.g., 32768). (iii) In other framestransmitted by a non-QoS STA or control frames transmitted by a QoS STA,the duration/ID field may include a duration value defined per frametype. In a data frame or a management frame transmitted by a QoS STA,the duration/ID field may include a duration value defined per frametype. For example, B15=0 of the duration/ID field indicates that theduration/ID field is used to indicate a TXOP duration, and B0 to B14 maybe used to indicate an actual TXOP duration. The actual TXOP durationindicated by B0 to B14 may be one of 0 to 32767 and the unit thereof maybe microseconds (μs). However, when the duration/ID field indicates afixed TXOP duration value (e.g., 32768), B15 can be set to 1 and B0 toB14 can be set to 0. When B14=1 and B15=1, the duration/ID field is usedto indicate an AID, and B0 to B13 indicate one AID of 1 to 2007. Referto the IEEE 802.11 standard document for details of Sequence Control,QoS Control, and HT Control subfields of the MAC header.

The frame control field of the MAC header may include Protocol Version,Type, Subtype, To DS, From DS, More Fragment, Retry, Power Management,More Data, Protected Frame and Order subfields. Refer to the IEEE 802.11standard document for contents of the subfields of the frame controlfield.

FIG. 11 illustrates a CF (contention free)-END frame.

It is assumed that the CF-END frame is transmitted by a non-DMG(directional multi-gigabit, 11ad) STA for convenience of description.The CF-END frame may be transmitted to truncate a TXOP duration.Accordingly, a duration field is set to 0 in the CF-END frame. An RA(Receiver Address) field may be set to a broadcast group address. ABSSID field may be set to an STA address included in a relevant AP.However, in the case of a CF-END frame in a non-HT or non-HT duplicateformat, which is transmitted from a VHT STA to a VHT AP, anIndividual/Group bit of the BSSID field may be set to 1.

Example of HE PPDU Structure

Described in the following are examples of HE PPDU (high efficiencyphysical layer protocol data unit) format in a wireless LAN systemsupportive of 11ax.

FIGS. 12 to 16 show examples of HE PPDU.

HE-SIG A (or HE-SIG1) field is located next to L-Part (e.g., L-STF,L-LTF, L-SIG) and duplicated by 20-MHz unit like the L-Part. For theHE-SIG A field, DFT period of 3.2 us and subcarrier spacing of 312.5 kHzare usable. For example, assuming that MCS 0 is used, HE-SIG A field canbe configured with 2 symbols.

HE-SIG B may be omitted from SU PPDU and UL trigger based PPDU (e.g., ULPPDU transmitted based on a trigger frame), whereas HE-SIG A can beincluded in all HE PPDUs.

HE-SIG A includes common control information (e.g., BW, GI length, BSScolors, CRC, Tail, etc.) on STAs. HE-SIG A field includes informationfor analyzing HE PPDU and thus information included in the HE-SIG Afield may vary depending on the format of the HE PPDU (e.g., SU PPDU, MUPPDU, trigger based PPDU, etc.). For example, (i) in HE SU PPDU format,HE-SIG A field may include at least one of a DL/UL indicator, an HE PPDUformat indicator, a BSS color, a TXOP duration, a BW (bandwidth), anMCS, CP+LTF length, coding information, #of streams, STBC (e.g., whetherSTBC is used), Tx beamforming (TxBF) information, CRC, and Tail. In caseof HE SU PPDU format, HE-SIG B field can be omitted. (ii) In HE MU PPDUformat, HE-SIG A field may include at least one of a DL/UL indicator, aBSS color, a TXOP duration, a BW (bandwidth), MCS information of SIG Bfield, #of symbols of SIG B field, #of HE LTF symbols, an indicatorindicating whether full-band MU-MIMO is used, CP+LTF length, Txbeamforming (TxBF) information, CRC and Tail. (iii) In HE trigger basedPPDU format, HE-SIG A field may include at least one of a formatindicator (e.g., indicator indicating SU PPDU or trigger based PUDU), aBSS color, a TXOP duration, a BW, CRC and Tail.

In HE-SIG A, user allocation information, e.g., at least one of STA ID(e.g., PAID, GID, etc.), allocated resource information, and #of streams(Nsts) may be included as well as the aforementioned common controlinformation.

BSS color information included in HE-SIG A field is the information foridentifying BSS and has a length smaller than that of BSSID. Forexample, BSS color information can have 6-bit length, whereas BSSID hasa 48-bit length. STA can determine whether it is an intra-BSS frameusing BSS color information. Namely, the STA can identify intra BSS PPDUand inter BSS PPDU through BSS color information despite decoding HE-SIGA field only without decoding the whole HE PPDU.

Referring to FIG. 13, the HE-SIG B (or HE-SIG2) may be transmitted foreach OFDMA allocation. In the case of MU-MIMO, the HE-SIG B isidentified by an STA through SDM. The HE-SIG B may include additionaluser allocation information, for example, an MCS, coding information,STBC (Space Time Block Code) information and transmission beamforming(TXBF) information.

FIG. 14 illustrates another example of the HE PPDU. The HE-SIG B istransmitted following the HE-SIG A. The HE-SIG B may be transmittedthrough the full band on the basis of numerology of the HE-SIG A. TheHE-SIG B may include user allocation information, for example, STA AID,resource allocation information (e.g., allocation size), MCS, the numberof streams (Nsts), coding, STBC and transmission beamforming (TXBF)information.

FIG. 15 illustrates another example of the HE PPDU. The HE-SIG B may beduplicated per predetermined unit channel Referring to FIG. 15, theHE-SIG B may be duplicated per 20 MHz. For example, the HE-SIG B can betransmitted in such a manner that the same information is duplicated per20 MHz in 80 MHz bandwidth.

An STA/AP which has received the HE-SIG B duplicated every 20 MHz mayaccumulate the received HE-SIG B per 20 MHz channel to improvereliability of HE-SIG B reception.

Since the same signal (e.g., HE-SIG B) is duplicated and transmitted perchannel, the gain of accumulated signals is proportional to the numberof channels over which the signal is duplicated and transmitted toimprove reception performance. In theory, a duplicated and transmittedsignal can have a gain corresponding to 3 dB×(the number of channels)compared to the signal before duplication. Accordingly, the duplicatedand transmitted HE-SIG B may be transmitted with an increased MCS leveldepending on the number of channels through which the HE-SIG B isduplicated and transmitted. For example, if MCS0 is used for the HE-SIGB transmitted without being duplicated, MCS1 can be used for the HE-SIGB duplicated and transmitted. Since the HE-SIG B can be transmitted witha higher MCS level as the number of channels for duplication increases,HE-SIG B overhead per unit channel can be reduced.

FIG. 16 illustrates another example of the HE PPDU. Referring to FIG.16, the HE-SIG B may include independent information per 20 MHz channel.The HE-SIG B may be transmitted in a 1× symbol structure like the Legacypart (e.g., L-STF, L-LTF, L-SIG) and HE-SIG A. Meanwhile, a length of“L-STF+L-LTF+L-SIG+HE-SIGA+HE-SIGB” needs to be identical in allchannels in a wide bandwidth. The HE-SIG B transmitted per 20 MHzchannel may include allocation information about the corresponding band,for example, allocation information per user using the correspondingband, user ID, etc. However, the information of the HE-SIG B may varybetween bands because the respective bands support different numbers ofusers and use different resource block configurations. Accordingly, thelength of the HE-SIG B may be different for respective channels.

FIG. 17 illustrates an HE-SIG B padding method by which lengths beforeHE-STF (e.g., lengths to the HE-SIG B) become identical for respectivechannels. For example, the HE-SIG B may be duplicated by a paddinglength to align HE-SIG B lengths. As illustrated in FIG. 18, the HE-SIGB corresponding to a necessary padding length may be padded to theHE-SIG B from the start (or end) of the HE-SIG B.

According to an example, one HE-SIG B field can be transmitted when thebandwidth does not exceed 20 MHz. When the bandwidth exceeds 20 MHz, 20MHz channels may respectively transmit one of a first type HE-SIG B(referred to hereinafter as HE-SIG B [1]) and a second type HE-SIG B(referred to hereinafter as HE-SIG B [2]). For example, HE-SIG B [1] andHE-SIG B [2] may be alternately transmitted. An odd-numbered 20 MHzchannel may deliver HE-SIG B [1] and an even-numbered 20 MHz channel maydeliver HE-SIG B [2]. More specifically, in the case of a 40 MHzbandwidth, HE-SIG B [1] is transmitted over the first 20 MHz channel andHE-SIG B [2] is transmitted over the second 20 MHz channel. In the caseof an 80 MHz bandwidth, HE-SIG B [1] is transmitted over the first 20MHz channel, HE-SIG B [2] is transmitted over the second 20 MHz channel,the same HE-SIG B [1] is duplicated and transmitted over the third 20MHz channel and the same HE-SIG B [2] is duplicated and transmitted overthe fourth 20 MHz channel. The HE-SIG B is transmitted in a similarmanner in the case of a 160 MHz bandwidth.

As described above, the HE-SIG B can be duplicated and transmitted asthe bandwidth increases. Here, a duplicated HE-SIG B may befrequency-hopped by 20 MHz from a 20 MHz channel over which an HE-SIG Bof the same type is transmitted and transmitted.

HE-SIG B [1] and HE-SIG B [2] may have different content. However,HE-SIG-Bs [1] have the same content. Similarly, HE-SIG Bs [2] have thesame content.

According to an embodiment, HE-SIG B [1] may be configured to includeresource allocation information about only odd-numbered 20 MHz channelsand HE-SIG B [2] may be configured to include resource allocationinformation about only even-numbered 20 MHz channels. According toanother embodiment of the present invention, HE-SIG B [1] may includeresource allocation information about at least part of even-numbered 20MHz channels or HE-SIG B [2] may include resource allocation informationabout at least part of odd-numbered 20 MHz channels.

The HE-SIG B may include a common field and a user-specific field. Thecommon field may precede the user-specific field. The common field andthe user-specific field may be distinguished in a unit of bit(s) insteadof a unit of OFDM symbol(s).

The common field of the HE-SIG B includes information for all STAsdesignated to receive PPDUs in a corresponding bandwidth. The commonfield may include resource unit (RU) allocation information. All theHE-SIG Bs [1] may have the same content and All the HE-SIG Bs [2] mayhave the same content. For example, when four 20 MHz channelsconstituting 80 MHz are classified as [LL, LR, RL, RR], the common fieldof HE-SIG B [1] may include a common block for LL and RL and the commonfield of HE-SIG B [2] may include a common block for LR and RR.

The user-specific field of the HE-SIG B may include a plurality of userfields. Each user field may include information specific to anindividual STA designated to receive PPDUs. For example, the user fieldmay include at least one of an STA ID, MCS per STA, the number ofstreams (Nsts), coding (e.g., indication of use of LDPC), DCM indicatorand transmission beamforming information. However, the information ofthe user field is not limited thereto.

UL MU Transmission

FIG. 19 is an explanatory diagram of an uplink multi-user transmissionsituation according to an embodiment of the present invention.

As described above, an 802.11ax system may employ UL MU transmission. ULMU transmission may be started when an AP transmits a trigger frame to aplurality of STAs (e.g., STA1 to STA4), as illustrated in FIG. 19. Thetrigger frame may include UL MU allocation information. The UL MUallocation information may include at least one of resource position andsize, STA IDs or reception STA addresses, MCS and MU type (MIMO, OFDMA,etc.). Specifically, the trigger frame may include at least one of (i) aUL MU frame duration, (ii) the number of allocations (N) and (iii)information per allocation. The information per allocation may includeinformation per user (Per user Info). The information per allocation mayinclude at least one of an AID (AIDs corresponding to the number of STAsare added in the case of MU), power adjustment information, resource (ortone) allocation information (e.g., bitmap), MCS, the number of streams(Nsts), STBC, coding and transmission beamforming information.

As illustrated in FIG. 19, the AP may acquire TXOP to transmit thetrigger frame through a contention procedure to access media.Accordingly, the STAs may transmit UL data frames in a format indicatedby the AP after SIFS of the trigger frame. It is assumed that the APaccording to an embodiment of the present invention sends anacknowledgement response to the UL data frames through a block ACK (BA)frame.

FIG. 20 illustrates a trigger frame format according to an embodiment.

Referring to FIG. 20, the trigger frame may include at least one of aframe control field, a duration field, an RA (recipient STA address)field, a TA (transmitting STA address) field, a common informationfield, one or more Per User Info fields and FCS (Frame Check Sum). TheRA field indicates the address or ID of a recipient STA and may beomitted according to embodiments. The TA field indicates the address ofa transmitting STA.

The common information field may include at least one of a lengthsubfield, a cascade indication subfield, an HE-SIG A informationsubfield, a CP/LTF type subfield, a trigger type subfield and atrigger-dependent common information subfield. The length subfieldindicates the L-SIG length of a UL MU PPDU. The cascade indicationindicates whether there is transmission of a subsequent trigger framefollowing the current trigger frame. The HE-SIG A information subfieldindicates content to be included in the HE-SIG A of the UL MU PPDU. TheCP/LTF type subfield indicates a CP and HE LTF type included in the ULMU PPDU. The trigger type subfield indicates the type of the triggerframe. The trigger frame may include common information specific to thetype and information per user (Per User Info) specific to the type. Forexample, the trigger type may be set to one of a basic trigger type(e.g., type 0), beamforming report poll trigger type (e.g., type 1),MU-BAR (Multi-user Block Ack Request) type (e.g., type 2) and MU-RTS(multi-user ready to send) type (e.g., type 3). However the trigger typeis not limited thereto. When the trigger type is MU-BAR, thetrigger-dependent common information subfield may include a GCR(Groupcast with Retries) indicator and a GCR address.

The Per User Info field may include at least one of a user ID subfield,an RU allocation subfield, a coding type subfield, an MCS subfield, aDCM (dual sub-carrier modulation) subfield, an SS (spatial stream)allocation subfield and a trigger dependent Per User Info subfield. Theuser ID subfield indicates the AID of an STA which will use acorresponding resource unit to transmit MPDU of the UL MU PPDU. The RUallocation subfield indicates a resource unit used for the STA totransmit the UL MU PPDU. The coding type subfield indicates the codingtype of the UL MU PPDU transmitted by the STA. The MCS subfieldindicates the MCS of the UL MU PPDU transmitted by the STA. The DCMsubfield indicates information about double carrier modulation of the ULMU PPDU transmitted by the STA. The SS allocation subfield indicatesinformation about spatial streams of the UL MU PPDU transmitted by theSTA. In the case of MU-BAR trigger type, the trigger-dependent Per UserInfo subfield may include BAR control and BAR information.

NAV (Network Allocation Vector)

A NAV may be understood as a timer for protecting TXOP of a transmittingSTA (e.g., TXOP holder). An STA may not perform channel access during aperiod in which a NAV configured in the STA is valid so as to protectTXOP of other STAs.

A legacy STA which does not support 11ax (e.g., non-HE STA) supports oneNAV. An STA which has received a valid frame can update the NAV throughthe duration field of the PSDU (e.g., the duration field of the MACheader). When the RA field of the received frame corresponds to the MACaddress of the STA, however, the STA does not update the NAV. When aduration indicated by the duration field of the received frame isgreater than the current NAV value of the STA, the STA updates the NAVthrough the duration of the received frame.

FIG. 21 illustrates an example of NAV setting.

Referring to FIG. 21, a source STA transmits an RTS frame and adestination STA transmits CTS frame. As described above, the destinationSTA designated as a recipient through the RTS frame does not set a NAV.Some of other STAs may receive the RTS frame and set NAVs and others mayreceive the CTS frame and set NAVs.

If the CTS frame (e.g., PHY-RXSTART.indication primitive) is notreceived within a predetermined period from a timing when the RTS frameis received (e.g., PHY-RXEND.indication primitive for which MACcorresponds to the RTS frame is received), STAs which have set orupdated NAVs through the RTS frame can reset the NAVs (e.g., 0). Thepredetermined period may be(2*aSIFSTime+CTS_Time+aRxPHYStartDelay+2*aSlotTime). The CTS_Time may becalculated on the basis of the CTS frame length indicated by the RTSframe and a data rate.

Although FIG. 21 illustrates setting or update of a NAV through the RTSframe or CTS frame for convenience, NAV setting/resetting/update may beperformed on the basis of duration fields of various frames, forexample, non-HT PPDU, HT PPDU, VHT PPDU and HE PPDU (e.g., the durationfield of the MAC header of the MAC frame). For example, if the RA fieldof the received MAC frame does not correspond to the address of an STA(e.g., MAC address), the STA may set/reset/update the NAV.

As described above, in an existing wireless LAN system (e.g.,11a/b/g/n/ac) previous to 11ax, an STA maintains a single NAV, and theNAV is configured through a duration field of a MAC header. Namely, eachof a TXOP holder (e.g., Tx STA) and a TXOP responder (e.g., Rx STA)sends total TXOP information necessary for transmission/reception offrames in a manner that the total TXOP information is included in aduration field of a frame transceived between the TXOP holder and theTXOP responder. Third party STAs other than the TXOP holder or the TXOPresponder check a duration field exchanged between the TXOP holder andthe TXOP responder and defer a channel use until an NAV term byconfiguring/updating an NAV.

Meanwhile, in a currently discussed flax WLAN system, an STA can supporta plurality of NAVs. For example, an flax STA can maintain a regular NAVand an intra BSS NAV. The regular NAV is configured to protect atransmission opportunity of PPDU that is not identified as an inter-BSSPPDU or an intra-BSS. The intra BSS is configured to protect atransmission opportunity for PPDU from a BSS to which an STA belongs.

TXOP Duration Field of HE-SIG A

In an 11ax system supportive of HE PPDU, third party STAs may be unableto obtain TXOP duration information (e.g., duration field) included in aMAC header of MPDU. Here, there is a problem that the third party STAhas difficulty in performing NAV configuration/update correctly. Tosolve it, an HE STA can send TXOP duration information in a manner thatthe TXOP duration information is included in HE-SIG A.

Meanwhile, 15 bits (e.g., B0˜B14) in a duration field of a MAC headercan indicate duration information and about maximum 32.7 ms (0˜32767 us)can be indicated with 1 us granularity.

If the 15-bit duration information included in the duration field of theMAC header is transmitted in a manner of being included in the HE-SIG Aintactly, it causes a problem that signaling overhead of the HE-SIG Aincreases excessively. Although 15 bits can be regarded as a relativelysmall size in MPDU for payload transmission on a MAC layer, since HE-SIGA for common control information transmission on a physical layer is afield designed compact, the increase of 15 bites in the HE-SIG Acorresponds to a relatively large signaling overhead.

Hence, in order to minimize an overhead of HE-SIG A, a TXOP duration ofthe HE-SIG A has a bit size smaller than that of a MAC duration fieldbut is configured to have a larger time unit granularity.

For example, a TXOP duration of HE-SIG A is set to a 7-bit size and agranularity of a time unit may be 8 or 128 us. MSB (i.e., B0) in 7 bitsmay be used to indicate whether a time unit granularity is 8 or 128 us.Particularly, when B0=0, a TOXP duration value may be 8 us×(B1−B6). WhenB0=1, a TXOP duration value may be (512+128) us×(B1−B6).

A TXOP duration of HE-SIG A may be configured based on MAC duration. Forexample, a TXOP of HE-SIG A may include a maximum value configurable ina TXOP duration field of the HE-SIG A without exceeding the MACduration.

NAV Update Method for HE PPDU

Although the existing NAV update is performed on the basis of a durationfield of a MAC header, since a TXOP duration field of HE-SIG A isincluded as well as the duration field of the MAC header in case of HEPPDU, it is unclear to determine that an NAV update should be performedbased on which field. Therefore, it is necessary to newly define an NAVupdate method for HE PPDU.

Proposal 1: Case of Updating NAV Using TXOP Duration of HE-SIG A

For one example, an STA having received HE PPDU can perform an NAVupdate using a TXOP duration of HE-SIG A.

For instance, if an STA receives HE PDU and a valid TXOP duration ofHE-SIG A is greater than a current NAV value of the STA, the STA canperform an NAV update with the TXOP duration of the HE-SIG A.

Particularly, when an STA receives HE PDDU, if a valid TXOP duration ofHE-SIG A is greater than a current NAV value of the STA and an RA(recipient address) of a received frame fails to match a MAC address ofthe STA, the STA can perform an NAV update with the TXOP duration of theHE-SIG A.

More particularly, when an STA receives HE PDDU, if a valid TXOPduration of HE-SIG A is greater than a current NAV value of the STA anda received fame is not intended for the STA, the STA can perform an NAVupdate with the TXOP duration of the HE-SIG A. If one of a case (i) thatan RA of a received frame fails to meet a MAC address of the STA, a case(ii) that AID/PAID/address of the STA is not included in HE-SIG A/B ofDL MU PPDU, and a case (iii) that BSS color information included in theHE-SIG A of the HE PPDU fails to match BSS color of AP/BSS associatedwith the corresponding STA, the STA can determine that the receivedframe is not a frame intended for the corresponding STA, by which thepresent invention is non-limited.

Proposal 2: Case of Updating NAV by Preferentially Considering MACDuration of MAC Header

For one example, an STA having received HE PPDU can perform an NAVupdate using a duration field (i.e., MAC duration) of a MAC header.

As described above, a MAC duration of a MAC header has a granularitygreater than that of a TXOP duration of HE-SIG A. The MAC duration ofthe MAC header has a length of 15 bits and a granularity of a time unitis 1 us. On the contrary, the TXOP duration of the HE-SIG A has a lengthof 4˜9 bits (e.g., 7 bits) and may support a time unit granularity equalto or greater than 16 us. For instance, the TXOP duration may be 8 or128 us, by which the present invention is non-limited.

According to one embodiment of the present invention, if an STA obtainsboth a MAC duration of a MAC header included in HE PPDU and a TXOPduration of HE-SIG A (e.g., if both a PHY preamble and a header of a MACframe are successfully decoded), the STA can prioritize the MAC durationrather than a TXOP duration field of an HE-SIG A field. Namely, inupdating a NAV, the STA can consider the MAC duration morepreferentially than the TXOP duration of the HE-SIG A field. Forinstance, updating an NAV based on a TXOP duration of an HE-SIG A fieldlike the proposal 1 may be limited to a case that an STA fails to obtaina MAC duration.

The reason for prioritizing a MAC duration rather than a TXOP durationof HE-SIG A is described as follows. Namely, since a granularity of theMAC duration is finer than that of the TXOP duration of the HE-SIG A, aMAC duration value can indicate a real TXOP duration more accuratelythan a TXOP duration value of the HE-SIG A.

(1) Case of Performing an NAV Update According to MAC Duration

If an STA receives HE PDU and obtains a valid MAC duration, the STAignores a TXOP duration of HE-SIG A and is able to perform an NAV updateaccording to the valid MAC duration. Here, if the STA obtains the validMAC duration, it may mean that the STA obtains the TXOP duration of theHE-SIG A. This is because the STA should succeed in decoding of apreamble of a PHY layer including the HE-SIG A in order to decode a MAClayer frame header.

A process for the STA to perform the NAV update according to the validMAC duration may be performed similarly to the existing NAV updateprocess. For instance, if a MAC duration of a received frame is greaterthan a current NAV value of the STA and an RA of the received framefails to match a MAC address of the STA, the STA updates an NAV with theMAC duration. Meanwhile, in case that PSDU (physical layer service dataunit) does not have an RA field, an NAV update can be performed like (i)or (ii). If one of a case (i) that AID/PAID/address is not included inHE-SIG A/B of DL MU PPDU and a case (ii) that BSS color informationincluded in HE-SIG A of HE PPDU fails to match BSS color of AP/BSSassociated with the corresponding STA is met, the STA updates the NAV.

The present example shows that the NAV is updated if the STA received anunintended PPDU, by which the present invention is non-limited. In somecases, in case of receiving an intended PPDU, the STA may update the NAVusing Duration (e.g., a duration field included in a MAC header).

(2) Case of Performing NAV Update Using TXOP Duration

Moreover, when an STA receives HE PPDU, in case that a valid TXOPduration of HE-SIG A is greater than a current NAV value of the STA andthat the STA fails to obtain a valid MAC duration of PSDU (i.e., MACframe), if a received frame is not intended for the corresponding STA,the STA performs an NAV update using the TXOP duration of the HE-SIG A.

In case that the STA fails to obtain the valid MAC duration of the PSDU(i.e., MAC frame), it may correspond to the following cases for example,by which the present invention is non-limited. Such cases may include:(i) a case that the STA fails to decode all MPDUs included in PSDU dueto errors; (ii) a case that an HE-SIG B field does not exist because areceived PPDU is UL MU PPDU; (iii) a case that a MAC header does notinclude a MAC duration field (e.g., short MAC header); and (iv) a casethat after the STA has decoded HE-SIG A of PPDU correctly, the STAenters a Doze/Sleep state for power saving for a duration of thecorresponding PPDU.

If one of following cases, which include a case that an RA of a receivedframe fails to meet a MAC address of the STA, a case thatAID/PAID/address of the STA is not included in HE-SIG A/B of DL MU PPDU,and a case that BSS color information included in HE-SIG A of HE PPDUdoes not match BSS color of AP/BSS associated with the correspondingSTA, is met, the STA can determine that the frame is not intended forthe corresponding STA, by which the present invention is non-limited.

Thus, depending on whether the STA obtains at least one valid frame fromPSDU, the NAV update can be performed.

According to the present example, the NAV is updated if the STA receivesan unintended PPDU, by which the present invention is non-limited. Insome cases, the STA may update an NAV using a TXOP duration (e.g., aTXOP duration field included in HE-SIG A) in case of receiving anintended PPDU.

Meanwhile, in the aforementioned examples, only if a TXOP duration fieldis set to a valid value as duration information, the STA may be able toupdate an NAV using the TXOP duration field. For instance, a specificvalue (e.g., 127) in the TXOP duration field may be interpreted asinvalid duration information. The specific value may include a valuethat the TXOP duration field is set to 1 all (e.g., binary 1111111).

Particularly, as a case that the STA fails to obtain durationinformation of a MAC header, if a TXOP duration value of an HE-SIG Afield is not 127 and the TXOP duration value is greater than a currentNAV value, the STA updates an NAV using the TXOP duration value. On thecontrary, although the STA fails to obtain duration information of a MACheader, if a TXOP duration value of an HE-SIG A field is 127, the STAdoes not update an NAV.

(3) NAV Update of STA Maintaining Two NAVs

Meanwhile, the proposed items of (1) and (2) are applicable to a casethat an STA maintains two NAVs (e.g., intra-BSS NAV, inter-BSS NAV). Theintra-BSS NAV is an NAV updated by receiving a frame transmitted by anSTA of an intra-BSS. The inter-BSS NAV is an NAV updated by receiving aframe transmitted by an STA of the inter-BSS, and is updated even if aframe (e.g., CTS), which is not identifiable as an intra-BSS PPDU or aninter-BSS PPDU, is received. The inter-BSS NAV may be referred to as anon-intra-BSS NAV, a regular NAV or a basic NAV.

(i) If an unintended STA accurately receives HE-SIG A of an intra-BSS HEPPDU (e.g., HE PPDU of which BSS color field included in HE-SIG Amatches a color of BSS having the STA belong thereto) and fails toreceive any one of a valid frame (i.e., MPDU) within PSDU of thecorresponding HE PPDU, the unintended STA updates an intra-BSS NAV usinga TXOP duration of the HE-SIG A. If the unintended STA receives at leastone valid frame (i.e., MPDU) within a PSDU of a frame indicated as aninter-BSS frame, it updates an intra-BSS NAV using a duration field(i.e., MAC duration) in the frame. For instance, if a TA (transmitteraddress) or RA (receiver address) included in a MAC header of a PSDUmatches a BSSID of an AP associated with the corresponding STA, itupdate an intra-BSS NAV with a MAC duration value. Although BSS color ofHE-SIG A indicates an intra-BSS PPDU, if the TA or RA of the MAC headerdoes not match the BSSID, the STA does not update the intra-BSS NAV witha value of the duration field. In this case, the STA ignores a TXOPduration field of HE-SIG A included in the corresponding HE PPDU.

(ii) If an unintended STA accurately receives HE-SIG A of an inter-BSSHE PPDU (e.g., HE PPDU of which BSS color field included in HE-SIG Afails to match a color of BSS having the STA belong thereto) and failsto receive any one of a valid frame (i.e., MPDU) within PSDU of thecorresponding HE PPDU, the unintended STA updates an inter-BSS NAV usinga TXOP duration of the HE-SIG A. If the unintended STA receives at leastone valid frame (i.e., MPDU) within a PSDU of a frame indicated as aninter-BSS frame, it updates an intra-BSS NAV using a duration field(i.e., MAC duration) in the frame. In this case, the STA ignores a TXOPduration field of HE-SIG A included in the corresponding HE PPDU.

(4) Inter-BSS NAV Update Considering Spatial Reuse

Moreover, the NAV update methods proposed in (1)˜(3) may be limitedlyapplicable only if a condition for spatial reuse is not met. Forinstance, only if a power of a received OBSS PPDU (e.g., inter-BSS PPDU)is not lower than an OBSS PD level configured for an STA, the STA canupdate an inter-BSS NAV by applying an NAV update rule. If the power ofthe received OBSS PPDU is lower than the OBSS PD level (i.e., if aspatial reuse condition is met), the STA does not update a currentinter-BSS NAV of the STA with an NAV (e.g., TXOP duration/MAC duration)of the received frame despite that an NAV value of the received frame isgreater than the current inter-BSS NAV configured for the STA.

OBSS PD level is schematically described as follows. First of all, inorder to determine whether a frame detected by an STA is an inter-BSSframe or an intra-BSS frame, the STA can use BSS color information, MACaddress information of a MAC header, etc. if the detected frame isdetermined as the inter-BSS fame, the STA can use an OBSS PD (packetdetection) level in order to determine whether a medium is idle (i.e.,CCS procedure). The OBSS PD level is set to a value greater than a CCAlevel (i.e., minimum receive sensitivity level) used for an intra-frame.

Namely, the STA determines whether the medium is busy with reference toa higher CCA level for another BSS (OBSS) frame in order to raise aspatial ruse rate, and is able to update an NAV. For instance, if anRSSI value of an OBSS frame is smaller than an OBSS PD level, the STAcan determine that a channel is in idle state. ON the contrary, if areceived frame is a frame (e.g., intra-BSS frame) within a BSS to whichthe STA belongs, the STA determines whether the medium is idle byapplying a CCA level (e.g., minimum receive sensitivity level) of alower level.

Thus, since a more flexible CCS determination reference applies to anOBSS frame, it become less probable that a medium is determined as busydepending on an OBSS frame reception. So to speak, although an OBSSframe is received, it becomes more probable that an STA can use achannel. Therefore, a use of an OBSS PD level enables a more efficientspatial reuse.

Proposal 3: Case of Updating NAV Using Higher Value Selected from MACDuration and TXOP Duration

According to another embodiment of the present inve3ntion, based on agreater value of a MAC duration of a MAC header and a TXOP duration ofan HE-SIG A field, an STA may perform an NAV update.

When an STA receives HE PPDU and obtains valid MAC duration information,if a MAC duration is greater than a TXOP duration of HE-SIG A, the STAignores the TXOP duration of the HE-SIG A and is able to perform an NAVupdate according to a valid MAC duration. Here, if the STA obtains thevalid MAC duration, it may mean that the STA obtains the TXOP durationof the HE-SIG A. A process for the STA to perform the NAV updateaccording to the valid MAC duration may be performed similarly to anexisting NAV update process. For instance, if a MAC duration of areceived frame is greater than a current NAV value and an RA of thereceived frame fails to match a MAC address of an STA, the STA updatesan NAV with the MAC duration.

If an STA receives HE PPDU, if the STA obtains valid MAC durationinformation, if a TXOP duration of HE-SIG A is greater than a MACduration, if a TXOP duration of the HE-SIG A is greater than a currentNAV value of the STA, and if a received frame is not intended for theSTA, the STA updates an NAV through the TXOP duration of the HE-SIG A.If one of a case (i) that an RA of a received frame fails to meet a MACaddress of the STA, a case (ii) that AID/PAID/address of the STA is notincluded in HE-SIG A/B of DL MU PPDU, and a case (iii) that BSS colorinformation included in the HE-SIG A of the HE PPDU fails to match BSScolor of AP/BSS associated with the corresponding STA, the STA candetermine that the received frame is not a frame intended for thecorresponding STA, by which the present invention is non-limited.

FIG. 22 and FIG. 23 are flowcharts of an NAV updating method accordingto one embodiment of the present invention. Substance redundant with theformer description can be omitted. For clarity, assume that the processof FIG. 22 is performed by an STA, by which the present invention isnon-limited. And, it is apparent to those skilled in the art that thesame process can be performed by an AP.

First of all, referring to FIG. 22, an STA receives a frame [2205]. Theframe may include HE PPDU. For instance, the received frame may includeHE SU PPDU, HE MU PPDU or HE trigger-based PPDU, by which the presentinvention is non-limited. A physical layer preamble of the receivedframe may include a legacy preamble part (L-part) and an HE preamblepart (HE-part). The HE-preamble may include at least one HE signalfield. For instance, the HE-preamble may include an HE-SIG A fieldand/or an HE-SIG B field. Yet, the HE-SIG B field may be omitteddepending on a type of the HE PPDU.

The frame may indicate a duration on each of a signal field (e.g.,SIG-A) and a header of a MAC layer. For instance, the frame may indicatedurations through a TXOP duration field of the HE-SIG A field and aduration field of the MAC header, respectively. Here, a duration (e.g.,TXOP duration) indicated on the physical layer may have a granularitygreater in a time unit than that of a duration (e.g., MAC duration)indicated on the MAC layer. Particularly, the TXOP duration may havegranularity of 8 or 128 us, and the MAC duration may have granularity of1 us.

The STA obtains the TXOP duration from the SIG-A field of the receivedframe [2210]. For instance, the STA attempts decoding of the receivedframe, thereby obtaining the TXOP duration from the SIG-A field.

As a result of the decoding attempt, if the STA obtains the MAC durationincluded in the MAC header as well as the TXOP duration, the STA ignoresthe TXOP duration and performs an NAV update using the MAC duration[2225]. On the other hand, if the STA fails to obtain the MAC durationincluded in the MAC header, the STA performs the NAV update using theTXOP duration [2220]. The case of failing to obtain the MAC duration mayinclude one of a case that the STA fails in decoding on all MPDUs (MACprotocol data units) included in the frame, a case that the receivedframe is an uplink frame, a case that the header of the MAC layer is ashort MAC header type, and a case that the STA makes transition to adoze mode for power saving after decoding the signal field, by which thepresent invention is non-limited.

Moreover, the NAV update 2220/2250 may be performed if the frame is notintended for the STA. For instance, if an RA (recipient address) of theframe mismatches a MAC address of the STA, if AID/PAID (associationidentifier/PAID) of the STA is not included in the signal field, or ifBSS (basic service set) color included in the signal field mismatches acolor of a BSS to which the STA belongs, the STA can determine that theframe is not intended for the STA.

With reference to FIG. 23, an NAV update process is described in detail.And, the following is apparent to those skilled in the art. First ofall, if the process of FIG. 23 applies to the NAV update step 2220, avalue used as a duration is a TXOP duration. Or, if the process of FIG.23 applies to the NAV update step 2225, a value used as a duration is aMAC duration.

When an STA updates an NAV, the STA selects one of a plurality of NAVsmaintained by the STA depending on whether a frame is received from aBSS (basic service set) to which the STA belongs and is then able toupdate the selected NAV. Here, a plurality of the NAVs may include anintra-BSS NAV and an inter-BSS NAV.

First of all, an STA determines whether a received frame is an intra-BSSframe [2305]. If the received frame is the intra-BSS frame and aduration of the frame is greater than an intra-BSS NAV value, the STAupdates the intra-BSS NAV with the corresponding duration [2315]. On thecontrary, if the duration of the frame is equal to or smaller than theintra-BSS NAV value, the STA maintain a current intra-BSS NAV valueintactly.

If the frame is an OBSS (overlapping BSS) frame, the STA determineswhether a power of the frame is smaller than an OBSS PD (packetdetection) level [2320]. The OBSS PD level is provided for spatial reuseand may have a value greater than a CCA (clear channel assessment)threshold applied to the intra-BSS frame.

If the power of the frame is smaller than the OBSS PD level, the STAmaintains a current inter-BSS NAV value [2330]. For instance, althoughthe duration of the frame exceeds a value of the inter-BSS NAV, theupdate of the inter-BSS NAV is not performed.

If the power of the frame exceeds the OBSS PD level and the duration ofthe frame exceeds the inter-BSS NAV value, the STA updates the inter-BSSNAV value with the duration of the frame [2335].

Despite that the power of the frame exceeds the OBSS PD level, if theduration of the frame is equal to or smaller than the inter-BSS NAVvalue, the STA maintains the current inter-BSS NAV value intactly[2330].

FIG. 24 illustrates devices for implementing the aforementioned methods.

A wireless device 100 and a wireless device 150 in FIG. 24 maycorrespond to the aforementioned specific STA and AP, respectively.

The STA 100 may include a processor 110, a memory 120, and a transceiver130 and the AP 150 may include a processor 160, a memory 170, and atransceiver 160. The transceivers 130 and 180 may transmit/receive awireless signal and may be implemented in a physical layer of IEEE802.11/3GPP. The processors 110 and 160 are implemented in a physicallayer and/or a MAC layer and are connected to the transceivers 130 and180. The processors 110 and 160 may perform the above-described UL MUscheduling procedure.

The processors 110 and 160 and/or the transceivers 130 and 180 mayinclude an Application-Specific Integrated Circuit (ASIC), a chipset, alogical circuit, and/or a data processor. The memories 120 and 170 mayinclude a Read-Only Memory (ROM), a Random Access Memory (RAM), a flashmemory, a memory card, a storage medium, and/or a storage unit. If anexample is performed by software, the above-described method may beexecuted in the form of a module (e.g., a process or a function)performing the above-described function. The module may be stored in thememories 120 and 170 and executed by the processors 110 and 160. Thememories 120 and 170 may be located at the interior or exterior of theprocessors 110 and 160 and may be connected to the processors 110 and160 via known means.

The detailed description of the preferred examples of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the preferred examples, those skilled in the art willappreciate that various modifications and variations can be made in thepresent invention without departing from the spirit or scope of theinvention described in the appended claims. Accordingly, the inventionshould not be limited to the specific examples described herein, butshould be accorded the broadest scope consistent with the principles andnovel features disclosed herein.

INDUSTRIAL APPLICABILITY

The present invention has been described on the assumption that thepresent invention is applied to a wireless LAN system supporting HEPPDUs. However, the present invention is not limited thereto and can beapplied to various wireless communication systems including IEEE 802.11.

What is claimed is:
 1. A method of updating a network allocation vector(NAV) by a station (STA) in a wireless local area network (LAN) system,the method comprising: receiving a frame; and updating the NAV of theSTA when duration information is greater than a NAV value of the STA,wherein the duration information is included in a first field and asecond field, wherein the duration information included in the firstfield has a greater granularity of a time unit than that of the durationinformation included in the second field, wherein the NAV is updatedwith the duration information of the first field having the greatergranularity when the duration information is not obtained from thesecond field, and wherein if the frame is an overlapping basic serviceset (OBSS) frame and a power of the frame is smaller than an OBSS packetdetection (PD) level, an update of the inter-BSS NAV is not performedeven if the duration information of the frame exceeds a value of theinter-BSS NAV.
 2. The method of claim 1, the first field is included ina signal (SIG) field of a physical layer, and the second field isincluded in a Medium Access Control (MAC) header of a MAC layer.
 3. Themethod of claim 1, wherein when the duration information included in thesecond field is obtainable, the duration information included in thefirst field is ignored in the update of the NAV.
 4. The method of claim1, wherein the NAV is updated with duration information included in thefirst field when the duration information included in the second fieldis obtainable.
 5. The method of claim 2, wherein the case where the STAdoes not obtain the duration information included in the second fieldcorresponds to a case where the STA fails to decode all MAC protocoldata units (MPDUs) included in the frame, a case where the receivedframe is an uplink frame, a case where the header of the MAC layer is ashort MAC header type, or a case where the STA makes transition to adoze mode for power saving after decoding the signal (SIG) field.
 6. Themethod of claim 1, wherein in updating the NAV, the STA selects andupdates one of a plurality of NAVs maintained by the STA depending onwhether the frame is received from a basic service set (BSS) where theSTA belongs, and wherein the plurality of NAVs include an intra-BSS NAVand an inter-BSS NAV.
 7. The method of claim 1, wherein the OBSS PDlevel is for a spatial reuse and has a value greater than a clearchannel assessment (CCA) threshold applied to an intra-BSS frame.
 8. Themethod of claim 1, wherein the NAV update is performed when the frame isnot intended to the STA.
 9. The method of claim 2, wherein the STAdetermines that the frame is not intended to the STA if a recipientaddress (RA) of the frame is not identical to a MAC address of the STA,if an association identifier (AID) of the STA is not included in the SIGfield, or if a basic service set (BSS) color included in the SIG fieldis not identical to a color of a BSS where the STA belongs.
 10. Astation (STA) updating a network allocation vector (NAV) in a wirelesslocal area network (LAN) system, the STSA comprising: a receiver forreceiving a frame; and a processor for updating the NAV of the STA whenduration information's greater than a NAV value of the STA, wherein theduration information is included each of a s first field and a secondfield, wherein the duration information included in the first field hasa greater granularity of a time unit than that of the durationinformation included in the second field, and wherein the NAV is updatedwith the duration information of the first field having the greatergranularity when the duration information is not obtained from thesecond field, and wherein if the frame is an overlapping bask serviceset (OBSS) frame and a power of the frame is smaller than an OBSS packetdetection (PD) level, an update of the inter-BSS NAV is not performedeven if the duration information Halite frame exceeds a value of theinter-BSS NAV.
 11. The STA of claim 10, the first field is included in asignal (SIG) field of a physical layer, and the second field is includedin a Medium Access Control (MAC) header a MAC layer.
 12. The STA ofclaim 10, wherein when the duration information included in the secondfield is obtainable, the duration information included in the firstfield is ignored in the update of the NAV.
 13. The STA of claim 10,wherein the NAV is updated with duration information included in thefirst field when the duration information included in the second fieldis obtainable.
 14. The STA of claim 11, wherein the case where theprocessor does not obtain the duration information included in thesecond field corresponds to a case where the STA fails to decode all MACprotocol data units (MPDUs) included in the frame, a case where thereceived frame is an uplink frame, a case where the header of the MAClayer is a short MAC header type, or a case where the STA makestransition to a doze mode for power saving after decoding the SIG field.15. The STA of claim 10, wherein in updating the NAV, the STA selectsand updates one of a plurality of NAVs maintained by the STA dependingon whether the frame is received from a basic service set (BSS) wherethe STA belongs, and wherein the plurality of NAVs include an intra-BSSNAV and an inter-BSS NAV.
 16. The STA of claim 10, wherein the OBSS PDlevel is for a spatial reuse and has a value greater than a clearchannel assessment (CCA) threshold applied to an intra-BSS frame. 17.The STA of claim 11, wherein the NAV update is performed when the frameis not intended to the STA and wherein the processor determines that theframe is not intended to the STA if a recipient address (RA) of theframe is not identical to a MAC address of the STA, if an associationidentifier (AID) of the STA is not included in the SIG field, or if abasic service set (BSS) color included in the SIG field is not identicalto a color of a BSS where the STA belongs.